1. Field of the Invention
This invention relates to a servo circuit of an optical disk information recording and reproducing apparatus that executes tracking control and/or focusing control for writing and/or reading information to or from the optical disk used as a record medium. More particularly, the invention is applied to an optical disk information recording and reproducing apparatus that writes and/or reads information to or from the optical disk wherein pre-grooves are provided and address signals are prerecorded on the respective land sections among these pre-grooves presenting physical changes in the form.
2. Description of the Prior Art
In a conventional optical disk information recording and reproducing apparatus, as shown in FIGS. 1 and 2, an optical disk 1 in which a record surface 2 is formed with a vertical magnetizing membrance having the opto-magnetic effect or Kerr effect for example is used as a record medium. In the optical disk 1, there are provided plural land sections each of which are formed between concentric pre-grooves G at equal intervals to serve as record tracks TR. One track is for example divided into 36 of 0 to 35 sectors, and each sector is further divided into a record area WA for writing information thereon and an address area AA for writing address signals thereon to designate the record area WA.
To the record area WA, a laser beam 10a which is modulated by recorded information is irradiated. Together with it, by imparting a magnetic field using a subsidiary magnetic field generating means such as a magnet so as to reverse the direction of magnetization of the recording surface 2 formed with the vertical magnetizing membrance having the opto-magnetic effect, signals are recorded to the record area WA. Also, in the address area AA, a pit row with the pits P in the form of projections and recesses designating the track address of the corresponding record area WA is previously provided presenting physical changes in the form. These pits P have the depth d of about .lambda./8 to .lambda./4 corresponding to the wavelength of the laser beam .lambda. used for writing and reading information.
Further, to a leading part of the address area AA, an address mark AM consisting of a pit pattern in the form of projections and recesses with the predetermined length is formed so as to indicate that this area is the address area.
In order to execute writing and reading information to the record area WA reliably, in the optical disk information recording and reproducing apparatus using the aforementioned optical disk 1 as a record medium, a servo circuit 30 is provided as shown in FIG. 3. For example in the servo circuit 30, focusing control or tracking control of the laser beam is executed in such a manner that a laser beam 10a radiated from a laser diode 10 irradiates the record surface 2 of the optical disk 1 through an optical head 20, and its reflected light 10b is detected therefrom.
The servo circuit 30 comprises a photodetector 31 with plural light receiving elements, a matrix circuit 32, a servo signal processing circuit 33 and a driving circuit 34. There are also provided a servo loop 37A for focusing control and a servo loop 37B for tracking control. In the respective servo loops 37A, 37B, focusing drive signals FD and tracking drive signals TD that are controlled based on the detection signals S.sub.1, S.sub.2 . . . Sn by said photodetector 31 such that focusing errors and tracking errors of the optical system 10 become zero are supplied to a focusing coil 21A and a tracking coil 21B, respectively.
The photo detector 31 consists of for example plural detecting elements D.sub.1, D.sub.2 . . . Dn, with RF signals and various servo signals being detected from the detection signals S.sub.1, S.sub.2 . . . Sn detected from said elements.
The matrix circuit 3 performs the predetermined arithmetic operating and processing on the detection signals S.sub.1, S.sub.2 . . . Sn by the photo detector 31 to output focusing error signals FE and tracking error signals TE.
The configuration for obtaining the focusing error signals FE is for example disclosed in the U.S. Pat. No. 4,023,033, and the one for obtaining the tracking error signals TE in the European Patent No. 0,201,603 although the detailed explanations of them are abridged since they differ from the essential part of the present invention.
The servo signal processing circuit 33 is formed by a focusing processing circuit 33A and a tracking processing circuit 33B. The focusing processing circuit 33A executes phase compensation on the focusing error signals FE so as to stabilize the operation of the servo loop 37A for focusing control. Similarly, the tracking processing circuit 33B executes phase compensation on the tracking error signals TE so as to stabilize the operation of the servo loop 37B for tracking control.
Further, the driving circuit 34 is formed by a focusing drive amplifier 34A and a tracking drive amplifier 34B. By amplifing the focusing error signals FE supplied from the focusing processing circuit 33A with phase compensation already been executed with an optimum loop gain, said focusing drive amplifier 34A forms the focusing drive signals FD which make focusing errors of the optical system zero and supplies them to the focusing coil 21A. Also, by amplifying the tracking error signals TE supplied from the tracking processing circuit 33B with phase compensation already been executed with an optimum loop gain, said tracking drive amplifier 34B forms the tracking drive signals TD which make tracking errors of the optical system 20 zero and supplies them to the tracking coil 21B.
In the servo circuit 30 with the above-mentioned configuration, the focusing error signals FE or tracking error signals TE are obtained by detecting the changes in the spot of the laser beam 10b reflected on the record surface 2 of the optical disk 1 by the photodetector 31. And by supplying the focusing drive signals FD and tracking drive signals TD that are formed based on the focusing error signals FE and tracking error signals TE respectively to the focusing coil 21A and the tracking coil 21B through the servo loops 37A, 37B, the light spots of the laser beam 10a radiated on the optical disk 1 are focused on the record surface 2. Together with it, focusing control and tracking control are performed such that the light spots of the laser beam trace the center of the tracks TR of the optical disk 1 that is rotating at a constant velocity.
In case the focusing control by the servo loop 37A or the tracking control by the servo loop 37B is performed with such conventional servo circuit 30, when the light spots of the laser beam tracking the record tracks TR formed on the record surface 2 of the optical disk 1 pass the address area AA having rows of the pits P in the form of projections and recesses presenting the physical changes in the form, there is caused an irregular light diffraction to the return laser beam 10b reflected by the record surface 2 due to these pits P of the address area AA. This results in an abrupt fluctuation of the outputs of the photodetector 31 (about 20dB).
Because of this, when the light spots of the laser beam stay in the address area AA, noise signals in the form of bursts are caused. At indicated in FIG. 4, this large impact noise is superimposed on the focusing drive signals FD or the tracking drive signals TD supplied to the focusing coil 21A and the tracking coil 21B, respectively.
In accordance with it, the focusing coil 21A and the tracking coil 21B are affected by such impact noise n to cause irregular and sudden fluctuations of the optical elements such as an object lens 20A operated at the focusing coil 21A or a galvanomirror 20B operated at the tracking coil 21B, both of which being components of the optical system 20, momentarily, to render the positional control of the optical system 20 temporarily difficult.
In order to eliminate the effect by the impact noise it may be contemplated to add a low-pass filter or use a secondary interpolating system for elimination or compensation thereof, respectively. However, the impact noise n extends over a wide range so that the servo circuit may be undesirably complicated or enlarged in size.